As a tank which is utilized for the storage of a hydrogen gas or the like, there is utilized a tank including a Fiber Reinforced Plastic (FRP) layer formed by alternately laminating hoop layers and helical layers on the outer periphery of a liner, thereby achieving weight saving (see e.g. Patent Document 1). The liner has a function of a member which prevents the permeation of the hydrogen gas or the like to store the gas or the like in an airtight manner, and the Fiber Reinforced Plastic (FRP) layer has a function of a member which imparts such a strength as to bear a high internal pressure.
The hoop layer which forms the Fiber Reinforced Plastic (FRP) layer is a layer formed by hoop-winding fibers (e.g. carbon fibers) (a winding way of winding the fibers around a tank trunk part almost vertically to a tank axis), and the helical layer is a layer formed by helically winding the fibers (a winding way of winding the fibers up to a tank dome part almost in parallel with the tank axis) (see FIG. 2). Moreover, as to the helical winding, when a winding angle with respect to the tank axis is changed, it is possible to obtain winding ways having different configurations, for example, high-angle helical winding and low-angle helical winding. When the Fiber Reinforced Plastic (FRP) layer is formed in this manner, how to wind the fibers is an important element for enhancing the efficiency of strength development by the Fiber Reinforced Plastic (FRP).
Heretofore, as a specific fiber winding way, it has been suggested that, for example, a peripheral fiber layer (a hoop layer and a high-angle helical layer) and an axial fiber layer (a low-angle helical layer) are alternately wound or successively wound so as to obtain a layer number ratio of 1:1.